In electrophotographic two-component system developers comprising a toner and a carrier, the electrical polarity of the toner, i.e., whether the toner should be charged positively or negatively, is selected according to the charging polarity of the photoreceptor to be used in the electrophotographic process. Such charging polarity of the toner depends chiefly on the kinds of substances constituting the toner and a carrier which triboelectrically charges the toner, and a substance located further from the positive side in the triboelectric series is negatively charged.
In recent years, organic electrophotographic photoreceptors having divided functions have been widely employed. Most of these organic electrophotographic photoreceptors are charged negatively in view of the mechanical nature thereof. Accordingly, positively chargeable toners have been demanded.
However, materials for positively chargeable toners are more limited than for negatively chargeable toners, making it difficult to design a toner composition for the negatively working photoreceptors.
Toners containing various additives have recently been proposed. Such additives include metallic or non-metallic oxides, e.g., SiO.sub.2, Al.sub.2 O.sub.3, SnO.sub.2, TiO.sub.2, etc., and they are markedly effective to improve powder fluidity, charging properties, and cleanability of toner particles, as described in Japanese Patent Application (OPI) Nos. 112052/85, 238847/85 and 66951/83 (the term "OPI" herein used refers to a "published unexamined Japanese Patent Application").
These conventional additives, however, cause many problems when applied to positively chargeable toners. For instance, use of hydrophilic alumina provides a developer which neither causes fog nor suffers abrupt deterioration of charging properties even when employed repeatedly for producing a number of copies, but the images obtained through repeated use become inferior in quality. More specifically, when the developer is used for a long period of time, a so-called "toner cloud" phenomenon occurs in the neighborhood of a developing machine to induce contamination in the developing machine. As a result, the optical system is contaminated with the toner particles to cause shortage of light quantity, providing images of low contrast, i.e., low density. Further, the toner particles drop in masses to cause stains on transfer sheets. On the other hand, addition of fine powders of tin oxide, hydrophobic silica or titanium dioxide to a toner reduces a usable life of the developer. That is, the developer suffers extreme deterioration of charging properties and some toner particles contained therein are charged to the opposite polarity to cause fog when repeatedly used for obtaining a large number of copies even in a short period of time.